The present application relates to an active material, a battery, and a method for manufacturing an electrode. Specifically, the present application relates to an active material containing composite particles including first particles with surfaces coated with second particles.
In recent years, many portable electronic apparatuses such as camcorders (Videotape Recorder), cellular phones, notebook-size personal computers, and the like have been developed, and reduction in size and weight has been attempted. Accordingly, lightweight secondary batteries capable of achieving a high energy density have been developed as power supplies for these electronic apparatuses. An example of the secondary batteries capable of achieving a high energy density is a lithium-ion secondary battery.
Recently, various studies have been made for improving the characteristics of lithium-ion secondary batteries. Studies on active materials are one of the particularly intensive research fields, and for example, the techniques described in Japanese Unexamined Patent Application Publication Nos. 11-273678, 2001-155729, 6-325791, 2008-153197, and 9-35715 and Japanese Patent No. 3232984 are disclosed.
Japanese Unexamined Patent Application Publication Nos. 11-273678, 2001-155729, 6-325791, and 2008-153197 disclose techniques for improving filling properties and battery characteristics by primary particles having a uniform particle size distribution and aggregates thereof. However, the primary particles have a large specific surface area, thereby decreasing a capacity retention (cycle properties). In addition, Japanese Unexamined Patent Application Publication No. 2008-153197 describes that the filling properties are adversely affected by aggregation of particles.
Japanese Patent No. 3232984 discloses a technique for improving battery characteristics by mixing secondary particles including aggregates of many microcrystal grains and microcrystal grains prepared by grinding the secondary particles. However, like in Japanese Unexamined Patent Application Publication Nos. 11-273678, 2001-155729, 6-32579, and 2008-153197, this technique uses only one type of microcrystal grains and thus exhibits a large specific surface area, thereby decreasing the capacity retention. In addition, a grinding step is further added for granulation, thereby causing decrease in productivity.
Japanese Unexamined Patent Application Publication No. 9-35715 discloses a technique for increasing the energy density and improving cycle characteristics by aggregating fine particles on the peripheries of core particles. However, when the inventors formed a battery by the disclosed method, the filling properties were not improved from those before aggregation by coating the peripheries of core particles with aggregates of fine particles, and the discharging capacity (battery capacity) tended to decrease. In addition, since the particle shape described in Japanese Unexamined Patent Application Publication No. 9-35715 has a large specific surface area, the capacity retention tended to decrease.
Japanese Unexamined Patent Application Publication No. 11-273678 discloses a technique for improving battery characteristics by mixing fine primary particles with secondary particles thereof. However, when the inventors formed a battery by the disclosed method, the filling properties were degraded during pressing, and the discharging capacity tended not to be improved.